Mechanisms of crystallisation

Now how does crystallisation occur There can be very different starting conditions for crystallisation. The usual, and therefore most important, is during solidification of an isotropic melt. However, crystallisation can also occur in the solid state when glassy polymers are heated to just above their glass transition temperatures. The morphologies obtained by the two methods might well be different. 

The first part of the curve is an induction period in which time is required for the formation of nuclei. The steepest part is related to the growth of the spheruhtes. The final part is when the spherulites begin to touch each other and the rate slows down. When the crystallisation temperature is increased, the shape 

All the above processes require substantial times, so usually crystallisation is not completed before further chain reorientation becomes impossible. So entanglement and disordered regions remain in the system. This means that polymers are never perfectly crystalline. The internal energy is therefore not at the lowest state and it can be said that crystallisation is governed by kinetic criteria rather than by equilibrium thermodynamics. 

The coiled form of the polymer has a large number of the higher energy conformations because thermodynamics dictates that these are populated at high temperature. As the melt is cooled, the number of these conformations will be decreased and the proportion of the chain in the lower energy linear conformation wiU increase.The net effect is that the polymer coil wiU expand and become less coiled. In fact, the size of the lamellae will reflect the temperature at which crystal growth occurs. The lower the temperature at which crystallisation occurs, the longer the sections which are linear and the thicker the lamellae. 

So the morphology found in semi-crystalHne polymer solids is a reflection of the conditions used in the production of the solid these are the temperature of the melt from which cooling is initiated, the rate of cooling, and whether or not the liquid experiences shear or compressive forces during solidification. 

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Strickland-constable, R. F. Kinetics and Mechanism of Crystallisation (Academic Press, London, 1968). 

Similar approaches were applied for studying the crystallisation of magnesium-substituted A1PO of type-36 (MgAPO-36).188From the obtained data, it was apparent that there exist two types of microstructural region in which five types of P(nAl) (n = 2-4) units could be identified. A three-stage mechanism of crystallisation was proposed. 

Concurrently with the preparation of the phenyldiazonium chloride solution, prepare a cold suspension of sodium arsenite. Place 250 ml. of water in a 3-htre round-bottomed flask equipped with a mechanical stirrer. Heat the water to boding, add 125 g. of anhydrous sodium carbonate, and, as soon as the carbonate has dissolved, introduce 62 5 g. of pure arsenious oxide and 3 g. of crystallised copper sulphate with stirring. When all the solids have dissolved, cool the solution with stirring under a stream of tap water until the temperature has fallen to 15°. 

Phthalide. In a 1 litre bolt-head flask stir 90 g. of a high quality zinc powder to a thick paste with a solution of 0 5 g. of crystallised copper sulphate in 20 ml. of water (this serves to activate the zinc), and then add 165 ml. of 20 per cent, sodium hydroxide solution. Cool the flask in an ice bath to 5°, stir the contents mechanically, and add 73-5 g. of phthalimide in small portions at such a rate that the temperature does not rise above 8° (about 30 minutes are required for the addition). Continue the stirring for half an hour, dilute with 200 ml. of water, warm on a water bath imtil the evolution of ammonia ceases (about 3 hours), and concentrate to a volume of about 200 ml. by distillation vmder reduced pressure (tig. 11,37, 1). Filter, and render the flltrate acid to Congo red paper with concentrated hydrochloric acid (about 75 ml. are required). Much of the phthalide separates as an oil, but, in order to complete the lactonisation of the hydroxymethylbenzoic acid, boil for an hour transfer while hot to a beaker. The oil solidifles on cooling to a hard red-brown cake. Leave overnight in an ice chest or refrigerator, and than filter at the pump. The crude phthalide contains much sodium chloride. RecrystaUise it in 10 g. portions from 750 ml. of water use the mother liquor from the first crop for the recrystaUisation of the subsequent portion. Filter each portion while hot, cool in ice below 5°, filter and wash with small quantities of ice-cold water. Dry in the air upon filter paper. The yield of phthalide (transparent plates), m.p. 72-73°, is 47 g. 

Both polymers are linear with a flexible chain backbone and are thus both thermoplastic. Both the structures shown Figure 19.4) are regular and since there is no question of tacticity arising both polymers are capable of crystallisation. In the case of both materials polymerisation conditions may lead to structures which slightly impede crystallisation with the polyethylenes this is due to a branching mechanism, whilst with the polyacetals this may be due to copolymerisation. 

The commercial appearance of phenolic resins fibres in 1969 is, at first consideration, one of the more unlikelier developments in polymer technology. By their very nature the phenolic resins are amorphous whilst the capability of crystallisation is commonly taken as a prerequisite of an organic polymer. Crystallisability is not, however, essential with all fibres. Glass fibre, carbon fibre and even polyacrylonitrile fibres do not show conventional crystallinity. Strength is obtained via other mechanisms. In the case of phenolic resins it is obtained by cross-linking. 

An important subdivision within the thermoplastic group of materials is related to whether they have a crystalline (ordered) or an amorphous (random) structure. In practice, of course, it is not possible for a moulded plastic to have a completely crystalline structure due to the complex physical nature of the molecular chains (see Appendix A). Some plastics, such as polyethylene and nylon, can achieve a high degree of crystallinity but they are probably more accurately described as partially crystalline or semi-crystalline. Other plastics such as acrylic and polystyrene are always amorphous. The presence of crystallinity in those plastics capable of crystallising is very dependent on their thermal history and hence on the processing conditions used to produce the moulded article. In turn, the mechanical properties of the moulding are very sensitive to whether or not the plastic possesses crystallinity. 

Thebainone (Schopf), CigHjjOgN. This substance, which must be distinguished from Pschorr s thebainone (metothebainone of Schopf (see p. 248) ), is formed, along with the latter in the reduction of thebaine by stannous chloride in hydrochloric acid, and was isolated by Schopf and Hirsch. Its prior isolation by Pschorr, as confirmed by Morris and Small, has been referred to already. It crystallises with 0-5 HjO, has m.p. 151-2°, yields a hydriodide, m.p. 258-9°, methiodide, m.p. 223°, and an oxime, m.p. 185-6°. On catalytic hydrogenation it yields dihydrothebainone (LI), and can be degraded to 3 4 6-triacetoxyphenanthrene, m.p. 165-7°. On this basis formula (XLIX) is assigned to it. The mechanism of the formation of codeinone, thebainone and mefathebainone from thebaine is discussed by Schopf and Hirsch. ... 

Matsuhisa J, Goldsmith JR, Clayton RN (1978) Mechanisms of hydrothermal crystallisation of quartz at 250 C and 15 kbar. Geochim Cosmochim Acta 42 173-182... 

Estimadon of liquidus and solidus temperatures of oxide inclusions in steels. The deformation of inclusions in steels has significant consequences on the hot workability of steels as well as for the mechanical properties of the final product. In order to increase their deformability there are at least three strategies (Matsumiya et al. 1987) (1), Reduction of their melting point (2), deceleration of crystallisation and (3), reducing their flow stress. If the melting point can be reduced sufficiently so that some liquid is present at the hot-working temperature, the inclusions would be expected to deform easily. 

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